A number of devices have heretofore been proposed and made commercially available for indicating point-level of material in a storage container or vessel. For example, U.S. Pat. No. 3,625,058 discloses a device of this character in which a tuning fork is positioned at a predetermined height within the vessel at which material level indication is desired, and is coupled to a motor for vibrating the fork. When material is spaced from the fork, the fork is free to vibrate. However, when the material reaches the level of the fork and covers the fork, vibration is damped. Thus, the fact that the level of material in the vessel has reached the height of the fork may be detected as a function of vibration characteristics of the fork itself.
U.S. Pat. No. 3,834,235 discloses a device in which an optical probe is positioned at the desired height of material level detection in the vessel. Light energy is directed into the probe from externally of the vessel. When material is spaced from the probe within the vessel, the difference in indices of refraction between the probe material and air at the probe tip is such that the light energy is reflected back out of the vessel for detection. On the other hand, when material covers the probe tip, the refractive index differential at the probe tip is so altered that the light energy is directed into the material. Hence, the fact that material has reached the level of the probe tip may be detected externally of the vessel by absence of light energy reflected from the probe tip.
U.S. Pat. No. 4,392,032 discloses a device of the subject character in which the probe takes the form of a paddle positioned within the vessel at the desired height of material level detection, and is connected to a motor carried within a suitable housing externally of the vessel. When the material is spaced from the paddle, the paddle is free to rotate as driven by the motor. However, when the material reaches the level of the paddle, paddle rotation is retarded, and such retardation may be detected externally of the vessel.
U.S. Pat. No. 4,499,766 discloses a material level indicating device in which a capacitance probe is positioned within a vessel such that electrical characteristics at the probe vary as a function of dielectric properties of the material, which in turn vary as a function of material level. The probe is connected in an LC resonant circuit to an rf oscillator, and phase shift of the probe signal is monitored to indicate changes in material level.